1. Field of the Invention
The present invention relates to a flow meter and method for detecting a cable fault in a cabling of the flow meter.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flow meters, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flow meter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes including, for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation by cabling, such as two independent pairs of wires. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement.
When the flow conduit or conduits of a Coriolis flow meter are empty, then the phase difference between the two pickoff signals is ideally zero. In contrast, during normal operation, the flow through the flow meter induces a phase shift between the two pickoff signals due to the Coriolis effect. The phase shift is directly proportional to the material flow through the conduits. Therefore, by making an accurate measurement of the signal difference, the flow meter can accurately measure the mass flow rate.
A Coriolis flow meter typically uses coils to drive a flow conduit(s) and to measure resulting flow conduit vibrations. In many cases, the flow sensor apparatus (i.e., the flow conduit(s), pickoff sensors, and driver), is not integrally mounted with the transmitter electronics. A typical Coriolis meter includes 9 wires bundled in a cable between the transmitter/meter electronics and the flow sensor apparatus. The cabling typically includes 3 wires for a Resistance Temperature Detector (RTD) sensor, 2 wires for a first pickoff sensor, 2 wires for a second pickoff sensor, and 2 wires for a driver.
The cabling is typically connected in the field and by the customer. This can lead to problems in the cabling. Pairs of wires can be swapped. Wires can be mixed up. Bad terminal connections or a failed coil can result in an open circuit. For example, if a first pickoff sensor is connected in a first orientation and the second pickoff sensor is connected in a second, opposite orientation, then a measured phase shift during a zeroing operation will be excessively large. Similarly, where the wires connecting to the driver are switched, then an expected phase characteristic will not be observed and a feedback loop of the driver circuit may drive the response toward zero instead of driving the response to a fundamental frequency.
Another problem that can occur is a broken or unconnected wire between components. A broken or unconnected wire may not be detected until the unit is put into operation. Troubleshooting the problem at the customer's location is costly and time-consuming. In addition, the customer will experience downtime, expense, and frustration.
It is desirable for the transmitter to automatically determine if the sensor is wired correctly, and if not, correct for the encountered problem. Additionally, it is desirable to determine this independently of process variations.